The present invention is related in general to the field of semiconductor devices and processes and more specifically to the material and fabrication of leadframes for integrated circuit devices.
The leadframe for semiconductor devices was invented (U.S. Pat. Nos. 3,716,764 and 4,034,027) to serve several needs of semiconductor devices and their operation simultaneously: First of all, the leadframe provides a stable support pad for firmly positioning the semiconductor chip, usually an integrated circuit (IC) chip. Since the leadframe including the pads is made of electrically conductive material, the pad may be biased, when needed, to any electrical potential required by the network involving the semiconductor device, especially the ground potential.
Secondly, the leadframe offers a plurality of conductive segments to bring various electrical conductors into close proximity of the chip. The remaining gap between the (xe2x80x9cinnerxe2x80x9d) tip of the segments and the conductor pads on the IC surface are typically bridged by thin metallic wires, individually bonded to the IC contact pads and the leadframe segments. Obviously, the technique of wire bonding implies that reliable welds can be formed at the (inner) segment tips. Thirdly, the ends of the lead segment remote from the IC chip (xe2x80x9couterxe2x80x9d tips) need to be electrically and mechanically connected to xe2x80x9cother partsxe2x80x9d or the xe2x80x9coutside worldxe2x80x9d, for instance to assembly printed circuit boards. In the overwhelming majority of electronic applications, this attachment is performed by soldering. Obviously, the technique of soldering implies that reliable wetting and solder contact can be performed at the (outer) segment tips.
It has been common practice to manufacture single piece leadframes from thin (about 120 to 250 xcexcm) sheets of metal. For reasons of easy manufacturing, the commonly selected starting metals are copper, copper alloys, iron-nickel alloys, and invar. Leadframes made of iron-nickel alloys (for instance the so-called xe2x80x9cAlloy 42xe2x80x9d) are about six times more expensive than copper leadframes. The desired shape of the leadframe is etched or stamped from the original sheet. In this manner, an individual segment of the leadframe takes the form of a thin metallic strip with its particular geometric shape determined by the design. For most purposes, the length of a typical segment is considerably longer than its width.
Cost reduction pressures in semiconductor manufacturing have initiated a search for replacing the copper and Alloy 42 leadframe base metals with lower cost materials. The early proposal to select aluminum could not solve the problem of breaking through the stable, omnipresent and almost instantaneous aluminum oxide. In U.S. Pat. No. 3,932,685, issued on Jan. 13, 1976 (Flowers, xe2x80x9cAluminum Stabilization Process and Stabilization Solution thereforxe2x80x9d), a process for preparing aluminum leadframes for wire bonding during semiconductor device assembly is described; however, the difficult problem of preparing aluminum leadframes for solder attachment is not addressed.
The plating process for stainless steel in U.S. Pat. No. 4,604,169, issued on Aug. 5, 1986 (Shiga et al., xe2x80x9cProcess for Metal Plating a Stainless Steelxe2x80x9d) does not apply to aluminum and its heavy oxide layer. Silver layers are not desirable in semiconductor products (U.S. Pat. No. 4,529,667, issued on Jul. 16, 1985, Shiga et al., xe2x80x9cSilver-Coated Electric Composite Materialsxe2x80x9d). The corrosion resistance achieved by the plating methods in U.S. Pat. No. 4,601,958, issued on Jul. 22, 1986 (Levine, xe2x80x9cPlated Parts and their Productionxe2x80x9d) and U.S. Pat. No. 4,835,067, issued on May 30, 1989 (Levine, xe2x80x9cCorrosion Resistant Electroplating Process, and Plated Articlexe2x80x9d) cannot be transferred to aluminum because the proposed protective layers would not adhere to aluminum; the iron-based alloys of those patents (Alloy 42 and Kovar) are expensive leadframe materials.
The method to produce an aluminum-coated leadframe in U.S. Pat. No. 4,604,291, issued on Aug. 5, 1986 (Huang et al., xe2x80x9cMethod for Manufacture of Aluminum-Coated Leadframexe2x80x9d) requires high vacuum and is thus impractical for mass production; it also does not address the problem of solder attachment.
The method to fabricate solder bumps on aluminum pads of a silicon chip described in U.S. Pat. No. 5,795,619, issued on Aug. 18, 1998 (Lin et al., xe2x80x9cSolder Bump Fabricated Method Incorporate with Electroless Deposit and Dip Solderxe2x80x9d) employs a process flow starting with forming a zinc layer on the aluminum using a zinc displacement solution. The subsequent dip into a sodiumhydrophosphate solution containing nickel and copper produces a nickel-copper-phosphorus layer on the zinc layer. Using a flux, the subsequent dip into a solder bath forms a solder alloy for solder bumps on the silicon chip. The nickel layer produced by this method is rich in copper (up to 74 weight % copper) and brittle; if this layer were applied to a leadframe, it would not be suitable, because leadframes have to undergo the demanding process step of bending and forming the leads before they are attached by solder to a printed circuit board. This forming step requires ductile leads. Further, the described technique, if applied to a leadframe, would offer no protection against corrosion of the leads and would also not be recommendable for a semiconductor product having a leadframe encapsulated in a molding compound where metal-to-plastic adhesion, low mechanical stress, and corrosion protection are required.
An urgent need has therefore arisen for a low-cost, reliable mass production method for a leadframe having an aluminum base, ductile leads and a surface which is simultaneously solderable and bondable. The leadframe and its method of fabrication should be flexible enough to be applied for different semiconductor product families and a wide spectrum of design and assembly variations, and should achieve improvements toward the goals of improved process yields and device reliability. Preferably, these innovations should be accomplished using the installed equipment base so that no investment in new manufacturing machines is needed.
According to the present invention for a semiconductor integrated circuit (IC) leadframe, aluminum or aluminum alloy serves as the starting material, followed by a sequence of deposited layers consisting of zinc (providing adhesion to aluminum and to nickel), electroless nickel (protecting the zincate from the electroplating solution), an optional nickel alloy (providing corrosion protection), electroplated nickel (providing ductility and solderability), and, outermost, a noble metal (protecting the nickel surface from oxidation).
The present invention is related to high density ICs, especially those having high numbers of inputs/outputs, or contact pads, and also to devices in packages requiring surface mount in printed circuit board assembly. These ICs can be found in many semiconductor device families such as standard linear and logic products, processors, digital and analog devices, high frequency and high power devices, and both large and small area chip categories. The invention represents a significant cost reduction of the semiconductor packages, especially the plastic molded packages, compared to the conventional copper or iron-nickel alloy leadframes.
It is an aspect of the present invention to provide a technology for modifying the surfaces of aluminum-based leadframes to provide both ductility and solderability without requiring costly fabrication processes. The aspect is achieved by mass production processes based on reel-to reel manufacturing.
Another aspect of the invention is to reach these goals without cost of equipment changes and new capital investment and using the installed fabrication equipment base.
Another aspect of the present invention is to provide the aluminum leadframes so that the successful surface mount technologies based on bending the package leads and solder attaching lead portions without flux (or only very mild fluxes not requiring rinse steps) can be continued. The end user of the semiconductor product can continue to employ the installed assembly equipment base.
Another aspect of the present invention is to provide easy bondability under the conditions defined by fine pitch bonding.
Another aspect of the present invention is provide the aluminum leadframes so that they exhibit good adhesion to the molding compounds used in plastic packages, thus preventing moisture ingress and corrosion, and also provide corrosion protection of the leads.
These aspects have been achieved by the teachings of the invention concerning material selection and methods suitable for mass production. Various modifications have been successfully employed.
In the first embodiment of the invention, aluminum leadframes are produced with an intermediate layer of an alloy between nickel and a noble metal coupling the layers of electroless nickel and electroplated nickel.
In the second embodiment of the invention, aluminum leadframes are produced without this intermediate layer.
The technical advances represented by the invention, as well as the aspects thereof, will become apparent from the following description of the preferred embodiments of the invention, when considered in conjunction with the accompanying drawings and the novel features set forth in the appended claims.